1. Field of the Invention
The present invention relates to a deflection scanner used in an image forming apparatus.
2. Description of the Related Art
In recent years, an image forming apparatus such as a color printer has in many cases employed a tandem system of forming images of yellow, magenta, cyan and black in four photosensitive members, respectively, and forming a color image by sequentially stacking the images on an intermediate transfer member or a recording material. As for the image forming apparatus using the tandem system, the deflection scanner is known which is structured in the following way in order to miniaturize the main body and reduce the price. Specifically, the deflection scanner prepares a set of light-emitting points for generating a plurality of light beams having different wavelengths, on the same axis in a sub-scan direction, deflects the light beams emitted from each light source with a polygon mirror, respectively, and irradiates a scanning lens with the deflected light beam. Then, the deflection scanner separates the light beams which have passed through the scanning lens by using a dichroic mirror and a polarization beam splitter, and irradiates a plurality of photosensitive members (a plurality of surfaces to be scanned) with the split light beams, respectively. The deflection scanner which has an optical system having such a structure arranged symmetrically with respect to the polygon mirror of the center is described in Japanese Patent Application Laid-Open No. 2009-063739.
However, when the deflection scanner irradiates symmetrically both sides of the polygon mirror with the light beams, stray light which has been generated by the reflection of the light beam on a lens surface of the scanning lens of one optical system and the like passes through the side of the polygon mirror and may be incident on the optical system on the opposite side. In this case, if the stray light has reached an image surface (photosensitive member) of the optical system on the opposite side, the stray light causes an image failure. In order to prevent such passing of stray light, it has been considered to provide a light-shielding plate in the vicinity of the polygon mirror to prevent the light beam which has been generated in the one optical system from being incident on the other optical system. However, because the polygon mirror rotates at a high speed, if the light-shielding plate has been arranged in the proximity of the polygon mirror, wind noise results between the light-shielding plate and the polygon mirror, and accordingly it is necessary to place the light-shielding plate and the polygon mirror at some distance. Accordingly, a gap between the light-shielding plate and the polygon mirror cannot be completely bridged, and accordingly a part of reflected light results in passing between the light-shielding plate and the polygon mirror. Because of this, the image failure cannot be occasionally fully prevented.